The third generation partnership project (3GPP) is currently working on standardization of Release 12 of Long Term Evolution (LTE) concepts. The architecture of an LTE system is shown in FIG. 1, which illustrates logical interfaces between eNBs (X2) and between eNB and MME/S-GW (S1), including radio access nodes (eNBs or eNodeBs) and evolved packet core nodes (MME/S-GW). As can be seen, an S1 interface(s) connects eNBs to the MME/S-GW(s), while an X2 interface(s) connects peer eNBs.
The management system assumed in embodiments of inventive concepts is shown in FIG. 2. The node elements (NE), also referred to as eNodeB, are managed by a domain manager (DM), also referred to as the operation and support system (OSS). A DM may further be managed by a network manager (NM). Two NEs are interfaced using an X2 interface, whereas the interface between two DMs is referred to as an Itf-P2P interface. The management system may configure the network elements, as well as receive observations associated with features in the network elements. For example, DM observes and configures NEs, while NM observes and configures DM, as well as NE via DM.
By means of configuration via the DM, NM, and/or related interfaces, functions over the X2 and S1 interfaces can be carried out in a coordinated way throughout the RAN (Radio Access Network), eventually involving the Core Network, i.e. MME and S-GWs.
The physical layer transmission in LTE uses OFDM (Orthogonal Frequency-Division Multiplexing) in the downlink and DFT-spread (Discrete Fourier Transform spread) OFDM in the uplink. The basic LTE physical resource can thus be seen as a time-frequency grid as illustrated in FIG. 3, where each resource element corresponds to one subcarrier during one OFDM symbol interval.
In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms (milliseconds), each radio frame consisting of ten equally-sized subframes of 1 ms as illustrated in FIG. 4. A subframe is divided into two slots, each of 0.5 ms time duration.
The resource allocation in LTE is described in terms of resource blocks (RB), also referred to as physical resource blocks or PRBs, where an RB corresponds to one slot in the time domain and 12 contiguous 15 kHz subcarriers in the frequency domain. Two in time consecutive RBs represent an RB pair and corresponds to the time interval upon which scheduling operates.
Transmissions in LTE are dynamically scheduled in each subframe where the base station (also referred to as eNodeB or eNB) transmits downlink assignments/uplink grants to certain UEs via the physical downlink control channel (PDCCH), or the enhanced PDCCH (EPDCCH) introduced in LTE Rel.11. In LTE downlink, data is carried by the physical downlink shared channel (PDSCH) and in the uplink the corresponding link is referred to as the physical uplink shared channel (PUSCH). The PDCCHs are transmitted in the first OFDM symbol(s) in each subframe and spans (more or less) the whole system bandwidth, whereas EPDCCH is mapped on RBs within the same resource region as used for PDSCH. Hence, EPDCCH s are multiplexed in the frequency domain with the PDSCH and it may be allocated over the entire subframe. A UE that has decoded an assignment carried by a PDCCH, or EPDCCH, knows which resource elements in the subframe contain data aimed for the UE. Similarly, upon receiving an uplink grant, the UE knows upon which time/frequency resources it should transmit upon.
Demodulation of sent data requires estimation of the radio channel which is done using transmitted reference symbols (RS), i.e. symbols known by the receiver. In LTE, cell specific reference symbols (CRS) are transmitted in all downlink subframes and in addition to assisting downlink channel estimation, they are also used for mobility measurements performed by the UEs. LTE also supports UE specific RS, i.e. demodulation reference signals (DMRS), to assist channel estimation for demodulation purposes only and channel state information RS (CSI-RS) used for channel feedback purpose only.
FIG. 5 illustrates mapping of PDCCH and PDSCH and CRS on resource elements within an LTE downlink subframe. In this example, the PDCCHs occupy the first out of three possible OFDM symbols, so in this particular case the mapping of data carried by PDSCH could start already at the second OFDM symbol. Since the CRS is common to all UEs in the cell, the transmission of CRS cannot be easily adapted to suit the needs of a particular UE. This is in contrast to DMRS which means that each UE has reference signals of its own placed in the data region of FIG. 5 as part of PDSCH. In LTE, subframes can be configured as MBSFN (Multicast-broadcast single-frequency network) subframes which implies that CRS s are only present in the PDCCH control region.
The length of the PDCCH control region, which can vary on a subframe basis, is conveyed in the physical control format indicator channel (PCFICH). The PCFICH is transmitted within this control region, at locations known by UEs. After a UE has decoded the PCFICH, it thus knows the size of the control region and in which OFDM symbol the data transmission starts. The physical hybrid-ARQ indicator channel (PHICH) is also transmitted in the control region. This channel carries ACK/NACK responses to a UE to inform if the uplink data transmission in a previous subframe was successfully decoded by the base station or not.
In the black and white rendering of FIG. 5, the shading for cell specific RS blocks and control blocks may be difficult to distinguish. This figure shows the CRS for the case of four CRS ports at the eNB. The cell specific RS blocks are shown in the 2nd, 5th, 8th, 11th, and 14th rows (from the bottom to the top) of the 1st, 5th, 8th, and 12th columns (from the left to the right). The control blocks are shown in the 1st, 3rd, 4th, 6th, 7th, 9th, 10th, 12th, 13th, 15th, and 16th rows (from the bottom to the top) of the 1st column (from the left). The 2nd and 3rd columns (from the left) may be columns of control or data blocks depending on the length of the control region.
Interference mitigation on the transmitter side refers to methods that aim to coordinate the physical channel transmissions across cells to reduce/avoid severe interference. A simple example is when an aggressor base station occasionally mutes its transmissions on certain radio resources in order for a victim cell to schedule interference sensitive UEs on radio resources with reduced interference. LTE features to coordinate transmissions have been specified in the context of inter-cell interference coordination (ICIC) and coordinated multipoint transmissions (CoMP). In the case of ICIC, an eNB sends a message over the LTE inter-eNB interface X2 with coordination information that a receiving eNB can take into account when scheduling interference sensitive users. In the case of CoMP, a cluster of transmission points, or base stations, can jointly and synchronously transmit the same signals to a UE to increase the received power on the desired signals, or it can as in the ICIC case coordinate the transmissions to reduce/avoid inter-point interference.
Over the X2 interface, procedures have been defined to support exchange of information enabling interference coordination. One of such procedures is the X2 Load Indication procedure shown in FIG. 6.
The LOAD INFORMATION message carries a number of IEs related to load and utilization in the sending eNB's cell. Some of the information carried by this message are described below and specified in 3GPP TS 36.423 V12.0.0, “X2 Application Protocol,” December 2013:                UL Overload Interference Indication (OII) indicates per RB the interference level (low, medium, high) experienced by the indicated cell on all RBs.        UL High Interference Indication (HII) indicates per RB the occurrence of high interference sensitivity, as seen from the sending eNB.        Received Narrow Transmit Power (RNTP) indicates per RB whether DL transmission power is lower than the value indicated by a threshold.        Almost Blank Subframe (ABS) pattern indicating the subframes the sending eNB will reduce power on some physical channels and/or reduced activity.        
The X2 IEs OII, HII and RNTP were specified in LTE Rel.8 and represent methods for coordinating physical data channel transmissions in the frequency domain across cells. The ABS IE, however, was specified in LTE Rel.10 as a time domain mechanism to primarily protect reception of PDCCH, PHICH and PDSCH in the small cells by letting macro cells occasionally mute, or reduce transmit power on PDCCH/PDSCH in certain subframes. The eNB ensures backwards compatibility towards UEs by still transmitting necessary channels and signals in the ABS for acquiring system information and time synchronization.
In R1-141816, LS on Inter-eNB CoMP for LTE, Release 12, March 2014, 3GPP RAN1 agreed to base the Inter eNB CoMP solution on signaling of the following information over X2 in Rel-12 LTE:                One or more CoMP hypotheses, each comprising a hypothetical resource allocation associated with a cell ID, where the cell identified by the cell ID is not necessarily controlled by the receiving eNB                    How to react to a received CoMP hypothesis signaling is up to receiving eNB's implementation. E.g. accept or ignore, potentially sending a feedback, e.g. “yes/no” to the sending node.            RAN1 guidance to RAN3 on necessary granularity and rate of CoMP hypothesis in time/frequency domain:                            Signaling period: RAN1's recommendation is 5, 10, 20, 40, 80 ms or aperiodic                                    If aperiodic, a validity period for the information should be included                                                RAN3 to specify the exact periodicities taking into account limitation of existing X2 interface                                    Per RB with time granularity per cell                            Time granularity could be one or multiple subframe level                                                A benefit metric associated with one or more CoMP hypothesis/es, quantifying the benefit that a cell of the sender node expects in its scheduling when the associated CoMP hypothesis/es is assumed                    The range of benefit metric in the X2 message should be specified            The method of deriving the cell-specific benefit metric is up to each eNB implementation            RAN1 guidance to RAN3:                            Necessary time/frequency granularity and signaling period: Same as the associated CoMP hypothesis/es                                                RSRP measurement reports of one or more UEs                    RAN1 guidance to RAN3:                            Time domain granularity of the signaling: event triggered or periodic exchange, with periodicities 120, 240, 480, 640 ms.                                    Mechanism to provide RSRP report upon request from an eNB should be made available                                                Per cell in sending eNB identified by cell ID:                                    Per UE identified by a UE ID, e.g. eNB-UE-X2-APID:                    One or more set(s) of {RSRP and cell ID} (maximum number of set(s) equals eight)                                                                                Note: CoMP signaling needs to be associated with a carrier frequency identity.        
The X2 interface, like the S1 interface, supports two types of procedures. They are defined in 3GPP TS 36.423 V12.0.0, “X2 Application Protocol,” December 2013, as below:                Elementary Procedure: X2AP protocol consists of Elementary Procedures (EPs). An X2AP Elementary Procedure is a unit of interaction between two eNBs. An EP consists of an initiating message and possibly a response message. Two kinds of EPs are used:                    Class 1: Elementary Procedures with response (success or failure),            Class 2: Elementary Procedures without response.                        
Class one procedures are typically used for functions that require confirmation from the receiving node of reception of a message and acknowledgement of an assumed behavior, or response with certain related information.
Class two procedures are typically used for functions where the sending node does not necessarily need to know of a behavior assumed by the receiving node and/or for functions where the information sent by the sending node have a limited life span and would require updating within a relatively short amount of time.
3GPP is currently working on support for Inter eNB CoMP for LTE with non Ideal Backhaul. An agreement has been taken in 3GPP to base solutions for Inter eNB CoMP on the use of the X2 interface and the new Rel-12 X2 signaling is described above. Accordingly, the continues to exist a need in the art for methods and devices providing improved interference mitigation.